Environmental and economical concerns associated with depleting oil resources have triggered a growing interest in the chemical conversion of carbon dioxide (CO2), so as to enable its use as a renewable carbon source. CO2 is, despite its low reactivity, a highly attractive carbon feedstock, as it is inexpensive, virtually non-toxic, abundantly available in high purity and non-hazardous. Therefore, CO2 could be a promising substitute for substances such as carbon monoxide or phosgene in many processes. One of the developing applications of CO2 is the copolymerization with epoxides to yield aliphatic polycarbonates, a field pioneered by Inoue et al. more than 40 years ago (Inoue, S. et al, J. Polym. Sci., Part B: Polym. Lett. 1969, 7, pp 287).
In WO2009/130470, the contents of which are incorporated herein by reference in their entirety, the copolymerisation of an epoxide with CO2 using a catalyst of a class represented by formula (I) was described:

Among the epoxides employed in the copolymerization, cyclohexene oxide (CHO) has received special interest, as the product, poly(cyclohexene carbonate) (PCHC) shows a high glass transition temperature and reasonable tensile strength. Propylene oxide has also received interest as it produces a polymer (polypropylene carbonate, known as PPC) with elastomeric properties which are useful in film applications. Kember et at (Angew. Chem., Int. Ed., 2009, 48, pp 931 and Inorg. Chem., 2009, 48, pp 9535) recently reported an air-stable di-zinc acetate complex, coordinated by a macrocyclic ligand, falling within formula (I) above, which shows high catalytic activity, even at ambient CO2 pressure. The catalyst exhibits excellent copolymerization selectivity, giving high proportions of carbonate repeat units and low yields of cyclic cyclohexene carbonate (CHC) by-product. The di-zinc acetate complex is a rare example of a catalyst that is capable of high activity at ambient pressure (1 bar) of CO2, yielding PCHC of moderate molecular weight, with narrow polydispersity index (PDI), and reaching remarkably high turnover numbers (TON).
Polycarbonates such as PCHC or PPC, are useful building blocks in the preparation of various copolymeric materials. Polycarbonates produced by copolymerisation of an epoxide with carbon dioxide using a catalyst of the class represented by formula (I) are generally terminated at one end with at least one hydroxyl group and at the other end by a group corresponding to ligand X. To use these polycarbonates as building blocks in the formation of, for example, block copolymers, it is desirable for all termination to be by hydroxyl groups. In a linear polycarbonate, for example, this would allow direct coupling of further polymer blocks to the ends of the polycarbonate, or growth of further polymer from the ends, i.e. by ring opening polymerisation, initiated by the terminal hydroxyl groups. Thus, further purification/work-up steps may be needed to replace terminal X groups with hydroxyl groups. A method for producing polycarbonates which are terminated with hydroxyl groups, avoiding the need for purification/work-up steps is desirable and it has been determined that this can be achieved by use of a chain transfer agent (CTA) during the polymerisation of CO2 with an epoxide.